I. Field of the Invention
The present invention relates generally to nuclear magnetic resonance (NMR) imaging methods employing multiple spin echo pulse sequences together with either multiple angle projection methods or two-dimensional Fourier transform (2DFT), or spin warp, imaging methods. In particular, the imaging methods involve the generation of multiple spin echoes induced by a repetitive sequence of phase alternated 180.degree. nonselective RF pulses. The multiple spin echo signals thus produced are used to improve the signal to noise ratio of the resulting NMR image, and/or advantageously to generate NMR images whose intensity reflects the transverse relaxation time T.sub.2 parameter alone.
II. Description of the Prior Art
NMR imaging as a medical diagnostic tool offers a number of important advantages over various other means available for probing the human body. The most significant of these advantages result from the completely noninvasive nature of the echnology, and the ability to obtain spatially encoded specimen data with a good degree of precision. Additionally, NMR has minimal, if any, hazards for either patients or operators of the apparatus; and perhaps most importantly, NMR image intensities are increasingly being found to be sensitive to various disease states. Clinical studies now underway are noting that the NMR relaxation times of malignant tissues are in general longer than those of the tissues of origin. This property is apparently not unique to cancerous tissue, but rather is indicative of the changes in molecular level structure of water associated with certain disease states. Other pathologies detected by NMR imaging include hydrocephalis; infarcted tissue; edema; multiple sclerosis plaques; and liver cirrhosis.
Accordingly, reference is now made herein to a recent article titled "Nuclear Magnetic Resonance: Beyond Physical Imaging" by Paul A. Bottomley, IEEE Spectrum, Vol. 20, no. 2, pp 32-38 (1983). More complete treatments of basic NMR concepts are provided in a recent text edited by Leon Kaufman et al., entitled "Nuclear Magnetic Resonance Imaging and Medicine", Igaku-Shoin, New York and Tokyo (1981); and also in an earlier text by Thomas C. Farrar et al., entitled "Pulse and Fourier Transform NMR, An Introduction to Theory and Methods", Academic Press, New York (1971).
The terms NMR imaging and NMR zeugmatography apply to an increasing range of NMR methods wherein static magnetic fields (to produce polarization of nuclei) are combined with magnetic field gradients (to spatially encode the sample volume of interest) and with RF magnetic fields (to spatially reorient polarized nuclei) to provide pictures of the spatial distribution of various NMR properties. In the recent past, the technical and patent literature have burgeoned, and have reported results of successive advances of the field.
Generated NMR signals characteristically exhibit two distinct relaxation times: the longitudinal (or spin-lattice) relaxation time T.sub.1, and the transverse (or spin-spin) relaxation time T.sub.2. Both T.sub.1 and T.sub.2 fall in the range of about 0.04 to 3 seconds. Their measurement typically involves the application of either of two types of radio frequency (RF) magnetic field excitation pulses applied at the NMR frequency (the so-called Larmor frequency).
At equilibrium an ensemble of nuclear magnets generate a net nuclear magnetization M aligned with the direction of the applied static field B.sub.o, the direction being arbitrarily designated the z-axis of a Cartesian coordinate system. A 90.degree. RF pulse causes the magnetization M to depart 90.degree. from the direction of the B.sub.o field into the x-y plane defined by the x-axis and y-axis, of the Cartesian coordinate system. Similarly, a 180.degree. RF pulse causes the magnetization M to reverse direction by 180.degree. from its original direction (from the positive z-axis direction to negative z-axis direction for example). Following the excitation of the nuclei with RF energy, the absorbed energy is re-radiated as an NMR signal as the nuclei return to equilibrium. The energy is emitted in the form of a RF magnetic field.
Two known methods of measuring T.sub.2 are the single spin echo (SE) method which involves the application of a 90.degree.:.tau.:180.degree. NMR RF pulse sequence wherein .tau. denotes a selectively variable delay, and the Carr-Purcell-Meiboom-Gill (CPMG) method, which involves the application of a 90.degree.:.tau.:180.degree.:2.tau.:180.degree.:2.tau.:180.degree. . . . NMR pulse sequence, with phase alternated 180.degree. pulses. A number of imaging schemes utilize the single spin echo sequence for other reasons and thereby generate images whose intensities are dependent on T.sub.2 and other parameters. The use of multiple 180.degree. RF pulses has been suggested specifically for the echo-planar NMR imaging method of P. Mansfield (see P. Mansfield et al. J. Magn. Reson., 29, 355, 1978). However, the approach of Mansfield et al. applies to a different NMR imaging sequence than disclosed herein, and is not for the purpose of improving the signal to noise ratio, nor for performing imaging of the T.sub.2 parameter explicitly, and is thus not deemed to perform the same function as the method described herein.
For medical applications of NMR imaging it is desirable to increase signal to noise ratio, shorten imaging time, enhance spatial resolution and image the transverse and/or longitudinal relaxation times, in order to increase the amount of useful information gained from NMR studies performed on each patient.
The present invention teaches the use of new NMR imaging pulse sequences based on improvements related to previously known sequences, which sequences are specifically directed to producing either a substantial improvement in the heretofore available signal to noise ratio, or to producing an image responsive largely to the transverse relaxation time T.sub.2, or both.